People usually use an indirect connection between the motor and the pinion gear to gain some mechanical advantage, and to facilitate spring-loading the gear into the rack. Here's an example (which you can purchase as an assembly, rather than have to construct yourself): Standard Rack and Pinion Drive, NEMA 34 | CNCRouterParts While ball screws can potentially give you more precision than rack-and-pinion, they are also more delicate (as well as being more expensive) and aren't generally used in plasma cutters, since the process generates so much grit and debris that it's easy to ruin them, unless they're well-protected. R&P drives are more forgiving, although it's still advisable to mount them in a sheltered position to avoid a buildup of dust on them. Linear rails have similar issues; plasma cutters often use alternative systems like V-rails: http://www.bwc.com/products/componen...s/dualvee.html